WO 97/45396 discloses N-bis- and N-tris-[(1,2-dicarboxy-ethoxy)-ethyl]-amine derivatives including N-bis-[(1,2-dicarboxy-ethoxy)-ethyl]-aspartic acid (also called aspartic acid diethoxy succinate or AES), and the use of these derivatives as chelating agents of metals. These derivatives can be prepared by reacting a di- or triethanolamine with an alkali metal or alkaline earth metal salt of maleic acid in the presence of a catalyst such as a lanthanide compound or an alkaline earth metal compound. This is a typical Michael addition reaction.
A method to prepare N-[2-(1,2-dicarboxyethoxy)-ethyl]aspartic acid (EDODS) by La3+-catalyzed O-alkylation of maleic acid salts has been described by J. van Westrenen et al. in Recl. Tray. Chim. Pays-Bas., vol. 109, 1990, p. 474-478.
Various methods to separate lanthanum from product solutions have been described in the literature. For example the precipitation of lanthanum as oxalate has been described. Thus, the lanthanum(III) ion used as a catalyst can be separated from the oxalate precipitate by treating the precipitate with nitric acid or hydrochloric acid. Moreover, the lanthanum oxalate precipitate can after filtration be treated at high temperatures. At 400° C. the oxalate is calcined to form carbonate and at temperatures of about 800-900° C. lanthanum oxide is formed. Both products can be reused as catalysts.
Lanthanum is a known catalyst which may be used in the Michael addition wherein a hydroxyl group is typically O-alkylated by addition to maleate. Useful lanthanum(III) compounds are lanthanum maleate, lanthanum(III) nitrate, lanthanum(III) chloride, lanthanum oxide and lanthanum octanoate.
The aim of the present invention is to improve the recovery of lanthanum catalyst or other lantanoid catalysts from the preparation of aspartic acid diethoxy succinate.